1. Field of the Invention
This invention relates to diesel engine exhaust gas treatment and more particularly to the filtering of particulates from diesel engine exhaust gases using a catalyzed filter.
2. The Prior Art
The question of how best to reduce the levels of particulate matter expelled to the atmosphere in the exhaust gases of diesel engines is currently of considerable interest. In this connection, it is desired to develop efficient and practical devices for removing substantial portions of particulates from the exhaust gases in diesel engine exhaust systems before permitting the exhaust gases to escape to the atmosphere.
It is known in the art to provide diesel engines with an exhaust filter which traps particulates from the exhaust gas stream during engine operation. The filters are generally made of porous, solid materials having a plurality of pores extending therethrough and having small cross-sectional size, such that the filter is permeable to the exhaust gases which flow through the filters and are capable of restraining most or all of the particulates from passing through the filter with the gas. The restrained particulates consist generally of carbonaceous particulates in the form of soot particles. As the mass of collected particulates increases, the flow rate of the exhaust gas through the filter is usually impeded, whereby an increased back pressure is encountered within the filter and reduced engine efficiency results. At this point, the filter is either discarded as a disposable/replaceable element or removed and regenerated by burning the collected particles off at temperatures in excess of 510.degree. C. so that the filter can be reused.
There is a desire in the art to more simply regenerate the particulate filter by continuous burn-off or incineration of the soot particles as they are trapped in the filter. However, experience has shown that in normal diesel engine operation, the temperature in the exhaust system varies substantially under different conditions of engine load and speed and that the temperatures in the filter only occasionly reaches the 510.degree. C. temperature level required to incinerate the trapped particulate.
The art, e.g., West German Disclosure Publication DE No. 3141713 published May 11, 1983, has attempted to rectify this situation by the use of a filter which embodies a catalytic material which lowers the ignition temperature of the soot particles exposed to the hot exhaust gas stream, the catalyst being composed of silver vanadate on a transition metal oxide carrier such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and oxides of rare earth metals. U.S. Pat. No. 4,303,552 teaches a diesel exhaust catalyst comprised of a bulk catalytic material consisting of an element of the first transition series, silver or hafnium, and a noble metal catalyst e.g. platinum, palladium or chromium supported on a porous refractory inorganic oxide such as .alpha.-alumina, magnesia and silica.
There is a need in the art, and it is therefore an object of the present invention, to effect soot particle burn-off in the diesel engine exhaust particulate filter at normal engine operating exhaust gas temperatures.
It is a further object of the invention to provide means whereby the temperature at which soot particles may be incinerated can be lowered to a level more closely approximating the exhaust gas temperatures encountered in normal diesel engine operation whereby burn-off and incineration of the soot particles trapped in the filter may be readily achieved and excessive particulate build-up in the filter avoided without risk of damage to the filter.